There is a contrail in the upper left (labelled NZ152) oriented more or less west-east. I think this was made by Air New Zealand Flight NZ152 which arrived in Wellington at 2:30 pm that afternoon from Melbourne.

Then there are four contrails oriented northeast – southwest over Marlborough and Cook Strait. These are labelled 1, 2, 3, 4. Contrails 2 (and possibly 4) are casting shadows on the ground and sea. The shadows are labelled 2s and 4s.

The contrail furthest west (labelled 1) must have come from an aircraft which was on a different route from those that made the other three because it is not parallel with them. This contrail also extends southwest into north Canterbury.

The two furthest east appear to converge slightly towards the north. I suspect this is because the (westerly) wind was increasing to the south and so the contrails are being rotated anticlockwise a little.

I took some photos of these contrails from the roof of the MetService building in Kelburn, Wellington at 2:45 pm Monday 19 July 2010, just 25 minutes after this MODIS image. Also, MetService’s web-cam at Christchurch Airport caught the southern end of the easternmost contrail (labelled 1 in the MODIS image) as it moved across Canterbury Plains.

Two contrails over Wellington Harbour at 2:45 pm on Monday 19 July 2010. This view is towards the north. One contrail (the eastern one on the right) is partly obscured behind the lower cumulus cloud. These two contrails are the eastern most ones in the satellite image.These two contrails are the western two in the satellite image. This photo was taken at 2:45 pm on Monday 19 July 2010.Christchurch Airport web-cam photos looking northeast. This is an animation of images at 10 minute intervals from 2 pm to 4:40 pm.

Around Wellington, the wind at the level of the contrails was quite a strong westerly, which explains why the contrails moved quickly across the sky from west to east. The wind in the atmosphere was also increasing with height; this spreads the contrails out enough to make them visible in satellite images. These contrails could still be seen in a lower resolution infra-red satellite image at 5:00 pm when they were 230 km east of Wellington.

In New Zealand, the long distance air routes are all more or less northeast – southwest. Thus, it is easy to see how several aircraft travelling these routes can create a set of parallel contrails. In Europe and North America the air routes are in all directions, and regular grid patterns of contrails are sometimes seen. Sometimes, the contrails in these grid patterns spread out to form a big sheet of high cloud: cirrus or cirrostratus.

This was one of the best and most persistent contrail outbreaks I have observed for some time. Contrails are interesting to watch and sometimes, as with these, there are fascinating details and patterns in the ice clouds as they evolve.

News

Clouds come in many different types and are characterised and named according to both their shape and height in the atmosphere. While a single snapshot in time at a given location may only contain one type of cloud, there are many days when multiple cloud types can be observed in the sky at once. On Saturday, 3 July 2010, we had a nice example in satellite imagery of different cloud types sitting at different levels above Waikato.

Here’s what the satellite image looked like Saturday afternoon for southwestern parts of Waikato near Kawhia Harbour:

(Image courtesy of MODIS Rapid Response Project at NASA/GSFC.)

Looking at that image you can probably spot three different cloud types quite easily. The broad, flat-looking cloud near the top right of the image is an area of fog sitting on the ground. Above this, and casting shadows on the fog layer, are lines or “streets” of puffy looking cumulus cloud. Higher still are thin wisps of cirrus both over the sea and extending onto the land, some of which you can see through to the cumulus and coastline below.

To see how these clouds changed during the afternoon, here’s a short animation of hourly images covering the period from midday to 3pm:

You should be able to spot the area of fog slowly shrinking, while cumulus forms near the coast in warm updrafts over the land eventually spreading inland across Waikato as streets of cloud. The lower quality of this animation makes the small wisps of cirrus hard to spot.

So how do these cloud streets form?
One might be tempted to compare each individual cloud street to a smoking chimney, and that is perhaps partially true. The cloud continuously forms at a stationary point, over a coastal hill for example (acting like a chimney), and is then blown away by the wind – a southwesterly in this case. However, this doesn’t completely explain the situation as you might expect the cloud to form all the way along the coast, and then move inland as a solid layer, not as lines of cloud with gaps between them. The answer is clearly a little more complicated and is best illustrated by the following image from Wikipedia:

Clouds form when rising air cools and reaches its saturation point, causing the moisture in the air to condense into water drops. Here we see lines of rising air or updrafts between counter-rotating tubes or rolls of air that are aligned with the flow. In between the updrafts are lines of sinking air in which cloud can’t form. The result is narrow lines of cloud, separated by narrow lines of cloud-free air.

Note that this is quite a different process to lines of cloud that are often observed downstream and parallel to a mountain range and perpendicular to the wind flow, but that story can wait for another blog post …

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